LXR qemu/util/stats64.c

   1/*
   2 * Atomic operations on 64-bit quantities.
   3 *
   4 * Copyright (C) 2017 Red Hat, Inc.
   5 *
   6 * Author: Paolo Bonzini <pbonzini@redhat.com>
   7 *
   8 * This work is licensed under the terms of the GNU GPL, version 2 or later.
   9 * See the COPYING file in the top-level directory.
  10 */
  11
  12#include "qemu/osdep.h"
  13#include "qemu/atomic.h"
  14#include "qemu/stats64.h"
  15#include "qemu/processor.h"
  16
  17#ifndef CONFIG_ATOMIC64
  18static inline void stat64_rdlock(Stat64 *s)
  19{
  20    /* Keep out incoming writers to avoid them starving us. */
  21    atomic_add(&s->lock, 2);
  22
  23    /* If there is a concurrent writer, wait for it.  */
  24    while (atomic_read(&s->lock) & 1) {
  25        cpu_relax();
  26    }
  27}
  28
  29static inline void stat64_rdunlock(Stat64 *s)
  30{
  31    atomic_sub(&s->lock, 2);
  32}
  33
  34static inline bool stat64_wrtrylock(Stat64 *s)
  35{
  36    return atomic_cmpxchg(&s->lock, 0, 1) == 0;
  37}
  38
  39static inline void stat64_wrunlock(Stat64 *s)
  40{
  41    atomic_dec(&s->lock);
  42}
  43
  44uint64_t stat64_get(const Stat64 *s)
  45{
  46    uint32_t high, low;
  47
  48    stat64_rdlock((Stat64 *)s);
  49
  50    /* 64-bit writes always take the lock, so we can read in
  51     * any order.
  52     */
  53    high = atomic_read(&s->high);
  54    low = atomic_read(&s->low);
  55    stat64_rdunlock((Stat64 *)s);
  56
  57    return ((uint64_t)high << 32) | low;
  58}
  59
  60bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high)
  61{
  62    uint32_t old;
  63
  64    if (!stat64_wrtrylock(s)) {
  65        cpu_relax();
  66        return false;
  67    }
  68
  69    /* 64-bit reads always take the lock, so they don't care about the
  70     * order of our update.  By updating s->low first, we can check
  71     * whether we have to carry into s->high.
  72     */
  73    old = atomic_fetch_add(&s->low, low);
  74    high += (old + low) < old;
  75    atomic_add(&s->high, high);
  76    stat64_wrunlock(s);
  77    return true;
  78}
  79
  80bool stat64_min_slow(Stat64 *s, uint64_t value)
  81{
  82    uint32_t high, low;
  83    uint64_t orig;
  84
  85    if (!stat64_wrtrylock(s)) {
  86        cpu_relax();
  87        return false;
  88    }
  89
  90    high = atomic_read(&s->high);
  91    low = atomic_read(&s->low);
  92
  93    orig = ((uint64_t)high << 32) | low;
  94    if (value < orig) {
  95        /* We have to set low before high, just like stat64_min reads
  96         * high before low.  The value may become higher temporarily, but
  97         * stat64_get does not notice (it takes the lock) and the only ill
  98         * effect on stat64_min is that the slow path may be triggered
  99         * unnecessarily.
 100         */
 101        atomic_set(&s->low, (uint32_t)value);
 102        smp_wmb();
 103        atomic_set(&s->high, value >> 32);
 104    }
 105    stat64_wrunlock(s);
 106    return true;
 107}
 108
 109bool stat64_max_slow(Stat64 *s, uint64_t value)
 110{
 111    uint32_t high, low;
 112    uint64_t orig;
 113
 114    if (!stat64_wrtrylock(s)) {
 115        cpu_relax();
 116        return false;
 117    }
 118
 119    high = atomic_read(&s->high);
 120    low = atomic_read(&s->low);
 121
 122    orig = ((uint64_t)high << 32) | low;
 123    if (value > orig) {
 124        /* We have to set low before high, just like stat64_max reads
 125         * high before low.  The value may become lower temporarily, but
 126         * stat64_get does not notice (it takes the lock) and the only ill
 127         * effect on stat64_max is that the slow path may be triggered
 128         * unnecessarily.
 129         */
 130        atomic_set(&s->low, (uint32_t)value);
 131        smp_wmb();
 132        atomic_set(&s->high, value >> 32);
 133    }
 134    stat64_wrunlock(s);
 135    return true;
 136}
 137#endif
 138